It is the same game more or less, but missing the "boss levels" (Defense). But for the price, who can complain? :) It has ResearchNet so you can work w/new content on the go...
From the Zachtronics site:
SpaceChem Mobile is a direct port of SpaceChem, meaning it’s almost exactly the same, but with a few notable differences:
The controls have been reimagined for touch (an obvious change), including a new way to quickly place arrow instructions (hold a button, drag a path).
The toolbar has been moved to the top to make it easier to place instructions.
The main campaign does not include boss battles or the story and is easier to progress through (more optional pipelines, specifically).
Video recording and forum signature generation are not available.
It only costs $5.99!
ResearchNet is fully functional in SpaceChem Mobile and will receive new issues of the Journal of Reaction Engineering at the same time as the PC version of SpaceChem.